This invention relates to the field of controlling the flow of solids in liquid or other fluid suspensions. More particularly, the invention relates to a system for stopping and starting the flow of suspended solids using mechanical valves which are not in communication with the solids in the suspension when they are opened and closed.
Circulation of solid/fluid slurries through conduits or vessels, or both, presents some problems in control of flow which are not present in purely fluid systems. A number of valve designs are available for permitting and preventing the flow of liquids or gases which are capable of functioning over long periods of time with some efficiency. However, the presence of suspended solid particles renders the use of mechanical valve systems at least troublesome because the solids represent a source of abrasion and wear to these mechanical systems.
The necessity for circulating or transferring slurries and for controlling this circulation and the transfer of the solid particles arises in a number of contexts, such as waste water treatment, ion exchange resin beds and manufacturing processes involving supported immobilized enzymes. A particularly important application is the necessity for catalyst replacement in petroleum refining, reforming, and hydroprocessing. Because the reactions such catalysts control generate deposits which progressively inactivate the catalyst, the catalyst needs periodically to be replaced.
Multitudinous approaches to the resolution of this latter problem have been disclosed, including replacement of a reactor's entire content of catalyst where the reactor is operated as a fixed bed. While this approach is relatively simple from an engineering standpoint, it may not be as efficient as a "moving bed" approach. In this approach, the catalyst is continuously replaced, by withdrawal of portions of catalyst, either co-current with the direction of reactant feed (see, eg, U.S. Pat. Nos. 3,849,295 to Addison (removal of catalyst from moving bed reactor systems) and 3,785,963 to Boyd et al (withdrawing uniform amounts of solids from a movable bed)), or countercurrent to reactant feed (see, eg, U.S. Pat. No. 3,910,834 (where counterflow of catalyst also filters out solids in the feed). Other related approaches have used fluidized or ebullating beds (see, eg, U.S. Pat. No. 4,217,206 to Nongbri).
Some of the continuous or intermittent replacement systems such as those of the previous paragraph can be operated in fluidized mode while others employ dense phase transport. In a dilute phase or fluidized transport, the solid is in a suspension wherein the properties of the suspension resemble those of a fluid. In a dense phase mode, however, the suspended particles are carried along by the viscous drag of the moving fluid and they are thus not suspended in the fluid in the technical sense. In traditional dilute phase transport, eg, the manner customarily used for transporting grain, 85 to 95% of the suspension is the carrying fluid if the fluid is a gas; about 60-80% is fluid, if liquid is the carrier. In a dense phase mode, about 10 to 50 times less gas or liquid is present as carrying fluid.
Experience in the field of gas/solid transport (as opposed to liquid/solid transport) has shown that the flow of solids in such systems can be controlled by an L-valve. L-valves have not been commonly used in liquid/solid transport. Briefly, an L-valve is a 90.degree. bend in the pathway traversed by the suspension of solids having a vertical and horizontal component. The suspension moves by falling through the vertical component. The horizontal component is of sufficient length relative to the amount of free vertical space above it that, absent the behavior of the suspension as a fluid, the solids dropped through the vertical component would come to rest in a pile corresponding to the angle of repose for the particles. The "valve" will not permit the flow of solid particles past it unless there is sufficient gas pressure behind the direction of flow to dislodge the solid particles from thus resting at their angle of repose. Simply put, the valve is closed to solid flow as long as circulation of gas is terminated; the valve is open to the passage of solids when the flow of gas continues to suspend the particles.
The use of such valves in gas/solid systems has been disclosed in U.S. Pat. No. 4,202,673 relating to coal gasification systems. In this case, the gas flow behind the L-valve is directly regulated in order to control solid transfer. U.S. Pat. No. 2,723,883 discloses another approach to controlling the flow of solids through a 90.degree. angle by offsetting the pressure exerted by a column of solid, thus permitting the height of the column to regulate the discharge of solid through the offset.
The present invention utilizes the L-valve in liquid/solid systems, offers a novel means whereby both solid/liquid systems and solid/gas systems can be used to transport solid particles, whereby both dense phase and fluidized systems are usable in solid transfer, and whereby the control of the solids transport can be achieved without use of mechanical valves in contact with the solid particles, using the fluid of the system itself as a means of control. Thus, the problems of poor control and mechanical wear on the valves are obviated. This is not the case with the presently used systems for controlling solid flow in gases or liquids, such as lock hoppers (which are scored frequently when opened and closed), double valving systems, and let down valves (which are destroyed by high velocity solids).